Recent emergence of high-temperature superconducting (HTS) thick-film tape technology is expected to meet the cost, size and performance requirements of superconducting components needed for advanced power applications for the defense and commercial sectors. One of the major potential HTS applications is in the electric power industry.
The YBa2Cu3O7 and related ceramic materials (YBCO) have appropriate intrinsic properties in the liquid nitrogen temperature range. However, their properties are drastically affected by grain boundary misorientations. In order to provide high temperature and field applications, it is imperative that a biaxially textured, polycrystalline YBCO tape, or related article, be developed which contains few high angle grain boundaries.
A biaxially textured, flexible metal/alloy substrate is formed by conventional thermomechanical processing followed by epitaxial deposition of buffer layer(s), YBCO grown on such substrate often exhibited Jc's over 1 MA/cm2 at 77K. To date, the preferred buffer layers have been the combination of CeO2 and yttria stabilized zirconia (YSZ). However, these oxide buffer layers lack important properties, e.g., electrical and thermal conductivity and mechanical toughness. It has been a challenging engineering task to develop a large-scale continuous process for producing thick (>0.5 μm) crack-and pore-free oxide films. Microcracking in oxide films is commonly observed in thick films due to the brittle nature of the oxide materials. Microcracks in the oxide layer can serve as open paths for oxygen to diffuse and oxidize the underlying metal during subsequent YBCO processing. Finally, the oxide deposition step on the Ni substrates is difficult; high quality films are only obtained by using very low deposition rates. In addition, as with many HTS applications, conductive buffer layers are needed since they would provide electrical coupling of the HTS layer to the underlying metallic tape substrate. This is an important property in order to electrically stabilize the conductor during transient loss of superconductivity in some applications.
Conventional ceramic fabrication methods which can be used to make a long, flexible conductor result in materials with weak, if any, macroscopic or microscopic biaxial texture. In particular, YBCO materials fabricated using conventional techniques invariably contain numerous high angle grain boundaries. High angle grain boundaries act as Josephson coupled weak-links leading to a significant field-dependent suppression of the supercurrent across the boundary. For clean stoichiometric boundaries, the grain boundary critical current density depends primarily on the grain boundary misorientation. The dependence of Jc(gb) on misorientation angle was first determined by Dimos et al. in YBCO for grain boundary types that can be formed in epitaxial films on bicrystal substrates. These include [001] tilt, [100] tilt, and [100] twist boundaries. In each case high angle boundaries were found to be weak-linked. The low Jc observed in randomly oriented polycrystalline HTS fabricated using conventional methods can be understood on the basis that the population of tow angle boundaries is small and that frequent high angle boundaries impede long-range current flow. Hence, controlling the grain boundary misorientation distribution towards low angles is key to fabricating high-Jc materials.
Successful fabrication of biaxially textured superconducting wire based on the coated conductor technology, requires optimization of the cost/performance of the HTS conductor. From a superconducting performance standpoint, a long, flexible, single crystal-like wire is required. From a cost and fabrication standpoint, an industrially scalable, low cost process is required. Both of these critical requirements are met by Rolling-assisted-biaxially-textured-substrates. However, in order for cost/performance for a conductor based on this technology to be optimized, further work needs to be done in the area of buffer layer technology. It is now clear that while it is fairly straight-forward to fabricate long lengths of biaxially textured metals or alloys, it is quite difficult to deposit high quality buffer layers using low cost processes. Requirements of buffer layers include—it should provide an effective chemical barrier for diffusion of deleterious elements from the metal to the superconductor, provide a good structural transition to the superconductor, have a high degree of crystallinity, excellent epitaxy with the biaxially textured metal template, have good mechanical properties, high electrical and thermal conductivity and should be able to be deposited at high rates.
Buffer layers of the prior art include use of YSZ and CeO2, typically a configuration of CeO2 (0.01 μm)/YSZ (0.5 μm)/CeO2 (0.01 μm). The purpose of the first buffer layer is to provide a good epitaxial oxide layer on the reactive, biaxially textured Ni substrate without the formation of undesirable NiO. CeO2 is special in its ability to very readily form single orientation cube-on-cube epitaxy on cube textured Ni. Deposition of CeO2 using a range of deposition techniques is done using a background of forming gas (4% H2–96% Ar) in the presence of small amounts of water vapor. Under such conditions the formation of NiO is thermodynamically unfavorable while the formation of CeO2 is thermodynamically favorable. The water vapor provides the necessary oxygen to form stoichiometric CeO2. It is not possible to deposit YSZ under such conditions with no evidence of undesirable orientations. In the case of CeO2 one can readily obtain a single orientation, sharp cube texture. Ideally, it would be desired that the CeO2 layer be grown thick such that it also provides a chemical diffusion barrier from Ni, followed by deposition of YBCO. However, when the CeO2 layer is grown greater than 0.2 μm in thickness, it readily forms micro-cracks. Hence, an YSZ that does provide an excellent chemical barrier to diffusion of Ni and does not crack when grown thick is deposited on a thin initial template of CeO2. However, since there is a significant lattice mismatch between YSZ and YBCO (˜5%), a second 45°-rotated orientation nucleates at times. In order to avoid the nucleation of this second orientation completely, a thin CeO2 layer is typically deposited epitaxially on the YSZ layer, to complete the buffer layer structure. YBCO can now be deposited on the layer that has an excellent lattice match with YBCO (˜0.1%).
The drawbacks of this buffer layer structure are that the deposition of the first CeO2 layer is non-trivial. Strict control of deposition conditions in particular, the O2 partial pressure is required to avoid formation of undesirable NiO (NiO typically nucleates in mixed orientations and is also very brittle). Furthermore, CeO2 can have wide range of oxygen stoichiometries. It is brittle and is not conducting. It will be a challenging engineering task to develop a large-scale continuous process for producing thick (>0.5 μm) crack-and porosity-free oxide films. For example, in a continuous process involving reactive electron beam evaporation of Ce to form CeO2, issues relating to the formation of an oxide on the target complicate matters relating to rate of deposition as well as stability of the melt pool. Any change of conditions during deposition is known to have profound affects on the film microstructure. Moreover, any oxidation of the biaxially textured metal, even after the successful deposition of CeO2, can induce undesirable interfacial stresses leading to spallation or further cracking, thus deteriorating the material properties. Microcracks in the oxide buffer layer will adversely affect the epitaxial quality of the growing YBCO film and create weak-links, besides serving as diffusion paths for Ni. Lastly, the surface morphology of the buffer layer is important for subsequent YBCO growth. Ideally, it would be desired to have a buffer layer which tends to be smoother than the Ni substrate it is grown on. All things considered, buffer layer deposition of the prior art is time-consuming and qualitatively deficient.
It has been an on-going concern in the art to meet the increasing demands for improved performance and miniaturization in next generation of electronic devices and components. New and advanced materials—primarily in the form of thin films—will be required. Deposition of oxide thin films is being pursued for a number of electronic applications including microelectronics (memory and microprocessing), sensors, fuel cells, superconductors, photonics, and other specialty markets. Oxide films provide protection against chemical attack, electrical and thermal insulation, and suitable dielectric properties, etc. However, as mentioned above, primary technical barriers in processing of oxide films include low deposition rates, poor film quality, and oxidation of substrate surfaces during deposition.
One approach to the aforementioned concerns has been to deposit alternate materials on the substrate to alleviate the mechanical deficiencies of the prior art oxides. Such materials are more robust, but often exhibit a lattice mismatch significantly detrimental to later deposition of a functional electromagnetic layer. Invariably, this and related difficulties are addressed with use of one or more suitable oxide layers. However, many of the aforementioned problems inherent to oxide films—slow deposition rates and micro-cracking, among them—remain and impede efficient, cost-effective buffer formation and device fabrication.
To illustrate, consider Lee, et al., Formation and Characterization of Epitaxial TiO2 and BaTiO3/TiO2 Films on Si Substrate. Jpn. J. Appl. Phys. Vol. 24, Pt. 1, No. 2B (February, 1995). Titanium dioxide thin films are described as converted from a nitride precursor. However, the resulting oxide is not biaxially textured; it is epitaxial out of plane but not in plane as would be necessary for superconducting applications. Biaxial texture can be defined in the context of superconducting applications as having a full-width—half-maximum (FWHM) less than about 15°, and preferably below 10°, for both in-plane and out of plane orientations. Further, the Lee films are described as having a rutile phase, presumably without an epitaxial lattice match as evidenced by x-ray results. The films are rough and porous, and exhibit high leakage and poor dielectric properties. Even so, there is no indication that this system could be extended, if desired, to deposit other materials on substrates more suitable to superconductor applications.
The foregoing background information, together with other aspects of the prior art, are disclosed more fully and better understood in light of the following publications:    1. Kormann, G., Bilde, J. B., Sorensen, K., de Reus, R., Anderson R. H., Vace, P., and Freltoft, T., “Relation between Critical Current Densities and epitaxy of Y-123 Thin Films on MgO (100) and SrTiO3 (100),” J. Appl. Phys., 1992, 71, 3419–3426.    2. Matsuda, H., Soeta, A., Doi, T., Aikhara, and T. Kamo, “Magnetization and Anisotropy in Single Crystals of Tl-1223 of Tl—Sr—Ca—Cu—O System,” Jpn. J. Appl. Phys., 1992, 31, L1229–31.    3. Dimos, D., Chaudhari, P., Mannhart, J., and F. K. LeGoues, “Orientation Dependence of Grain Boundary Critical Currents in YBa2Cu3Ox Bicrystals”, Phys. Rev. Lett., 1988, 61, 219–222; Dimos, D., Chaudhari, P., and Mannhart, J., “Superconducting Transport Properties of Grain-boundaries in YBa2Cu3Ox Bicrystals”, Phys. Rev. B, 1990, 41, 4038–4049.    4. Iijima, Y., Tanabe, N., Kohno, O., and Ikeno, Y., “In-plane Aligned YBa2Cu3Ox Thin-Films Deposited on Polycrystalline Metallic Substrates”, Appl. Phys. Lett., 1992, 60, 769–771.    5. Reade, R. P., Burdahl, P., Russo, R. E., and Garrison, S. M., “Laser Deposition of Biaxially Textured Yttria-stabilized Zirconia Buffer Layers on Polycrystalline Metallic Alloys for High Critical Current Y—Ba—Cu—O Thin-films”, Appl. Phys. Lett., 61, 2231–2233, 1992.    6. Wu, X. D., Foltyn, S. R., Arendt, P. N., Blumenthal, W. R., Campbell, I. H., Cotton, J. D., Coulter, J. Y., Hults, W. L., Maley, M. P., Safar, H. F., and Smith, J. L., “Properties of YBa2Cu3Ox Thick Films on Flexible Buffered Metallic Substrates”, Appl. Phys. Lett., 1995, 67, 2397.    7. Hasegawa, K., Fujino, K., Mukai, H., Konishi, M., Hayashi, K., Sato, K., Honjo, S., Satao, Y., Ishii, H., and Iwata, Y., “Biaxially Aligned YBCO Film Tapes Fabricated by All Pulsed Laser Deposition”, Appl. Supercond., 4, 475–486, 1996.    8. Goyal, A., Norton, D. P., Christen, D. K., Specht, E. D., Paranthaman, M., Kroeger, D. M., D. P., Budai, J. D., He, Q., List, F. A., Feenstra, R., Kerchner, H. R., Lee, D. F., Hatfield, E., Martin, P. M., Mathis, J., and Park, C., “Epitaxial Superconductors on Rolling-Assisted-Biaxially-Textured-Substrates (RABiTS): A Route Towards high Critical Current Density Wire”, Applied Supercond., 1996, 69, 403–427.; A. Goyal et al., U.S. Pat. No. 5,739,086 and U.S. Pat. No. 5,741,377.    9. Norton, D. P., Goyal, A., Budai, J. D., Christen, D. K., Kroeger, D. M., Specht, E. D., He, Q., Saffian, B., Paranthaman, M., Klabunde, C. E., Lee, D. F., Sales, B. C., and List, F. A., “Epitaxial YBa2Cu3Ox on Biaxially Textured Nickel (100): An Approach to Superconducting Tapes with High Critical current Density”, Science, 1996, 274, 755.    10. Goyal, A., Norton, D. P., Kroeger, D. M., Christen, D. K., Paranthaman, M., Specht, E. D., Budai, J. D., He, Q., Saffian, B., List, F. A., Lee, D. F., Hatfield, E., Martin, P. M., Klabunde, C. E., Mathis, J., and Park, C., “Conductors With Controlled Grain Boundaries: An Approach To The Next Generation, High Temperature Superconducting Wire”, J. Mater. Res., 1997, 12, 2924–2940.    11. Goyal, A., Norton, D. P., Budai, J. D., Paranthaman, M., Specht, E. D., Kroeger, D. M., Christen, D. K., He, Q., Saffian, B., List, F. A., Lee, D. F., Martin, P. M., Klabunde, C. E., Hatfield, E., and Sikka, V. K., “High Critical Current Density Superconducting Tapes By Epitaxial Deposition of YBa2Cu3Ox Thick Films on Biaxially Textured Metals”, Appl. Phys. Lett., 1996, 69, 1795.    12. W. D. Sproul, “Physics and Chemistry of Protective Coatings”, Ed. by W. D. Sproul, J. E. Greene, and J. A. Thornton (AIP Conf. Proc. No. 149, 1986, New York), p. 157.